1. Field
This disclosure relates to rocket motors.
2. Description of the Related Art
Solid fuel rocket motors are commonly used in various configurations to propel rockets and missiles. Small solid fuel rocket motors may also be used to control the attitude and steering of a missile, rocket, or other projectile.
The thrust or force produced by a rocket motor is given by the equationF=mp*Ue+(Pe−Pa)*Ae  (1)
where                mp=propellant mass flow rate, and        Pe=Nozzle exit pressure        Pa=Ambient pressure        Ae=Nozzle exit area        Ue=gas velocity at nozzle exit plane.The first term on the right side of equation (1) is the “momentum thrust” caused by the momentum of the exhaust flow from the rocket motor. The second term on the right side of equation (1) is the “pressure” thrust due to the pressure differential at the nozzle exit plane. Rocket motors are commonly designed such that the momentum thrust is the dominant component of the total thrust.        
In some circumstances, a rocket motor may be designed to have an “optimum expansion ratio” such that the nozzle exit pressure and the ambient pressure are equal. In this case, the thrust or force produced by a rocket motor is due to the momentum thrust only, and is given by the equationF=mp*Ue  (2)
The propellant mass flow rate mp is given by the equationmp=Ap*Rb*Pp  (3)
where                Ap=propellant surface area,        Rb=propellant burn rate, and        Pp=propellant density.        
Thus the propellant surface area Ap is one of the factors that may be used to control the thrust produced by a solid fuel rocket.
Insensitive munitions are munitions that minimize the probably of inadvertent ignition or detonation, and which minimize the severity of collateral damage to weapons platforms, other equipment and personnel if inadvertent ignition should occur. Specifically, an insensitive rocket motor is a motor that does not react more violently than burning when subjected to slow or fast heating; fragment, bullet, spall, or shaped charge impact; or detonation of an adjacent similar motor. Requirements for insensitive munitions are generally described in MIL-STD-2105B, Hazard Assessment Test for Non-Nuclear Ordinance. Specific test requirements are described in NATO STANAG (Standardization Agreement) documents.
Of particular interest for this patent are the “fast cookoff” and “slow cookoff” tests defined in STANAG 4240 and STANAG 4382. A fast cookoff test is intended to simulate an accidental fire aboard a ship or in an ammunition storage facility. During a fast cookoff test, the ordinance under test is suspended above a pool of burning aviation fuel. In a slow cookoff test, the temperature of the ordinance is increased at a rate of 3.3° C. per hour until a reaction occurs. To minimize the test time, the test may be started at a temperature 55° C. below the anticipated ignition temperature of the ordinance. In either test, the most severe reaction allowed is burning. In the case of a rocket motor, it is permissible for the fuel within the motor to burn but the motor may not produce thrust sufficient to allow the rocket to launch.
Insensitive rocket motors generally include a propellant material that does not ignite due to the impact of a bullet, fragment or shaped charge. Insensitive rocket motors generally also include some method to control the pressure within the rocket motor to prevent detonation and to limit the thrust produced by the rocket motors. A known way to limit the pressure within rocket motors is to include vents or other forms of pressure relief that are activated by high temperature or fire external to the rocket motor.
Throughout this description, elements appearing in figures, other than graphs, are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits.